He left the child's twin brother unharmed but attacked Leyton, who was suffering with teething and heartburn, Manchester Crown Court heard.

A catalogue of "disturbing" injuries included stamps and kicks to Leyton's genitals, inflicted after the twins' mother, Chantelle Flynn, 25, had left him to mind them at their Rochdale home while she was out with her other children.

Dawick, a cannabis and cocaine user who regularly beat up his partner, did not like to be woken up and would shout "Shut the f*** up!" if the babies cried, the court heard.

He denied murder but was convicted by a jury following a 12-day trial.

Flynn, who was "besotted" with Dawick, describing him as a "brilliant father", was convicted of causing or allowing the death of her baby and perverting the course of justice.

She knew Dawick was violent and a risk to her child but covered up for him, lying to paramedics and police that he was out when Leyton was injured.

Dawick blinked hard as he was convicted and Flynn, standing beside him in the dock, burst into tears.

Passing a mandatory life sentence for murder, Judge Sir John Royce, jailed Dawick for a minimum of 21 years before parole.

He told the defendant: "It is difficult to comprehend how any father could kick or stamp on his child's pubic area with such force to cause these gross injuries.

"It defies belief. Some might describe it as sadistic. This was a murder which borders on the sadistic.

Earlier the court heard Flynn had left the house on the morning of September 6 last year, leaving the twins with Dawick, who was playing on a computer.

Half an hour later he called her phone and when she arrived back at the house, Leyton was struggling to breathe and losing colour so she called an ambulance.

One trauma expert said Leyton's injuries were among the worst he had seen in more than 300 child injury cases and were normally associated with high-speed car crashes.

Some jurors covered their mouths with their hands as the catalogue of injuries was outlined during the trial.

They included fractures to his ribs, pubic bone area, pelvis, right wrist and right leg in three places - femur, shin bone and outer leg bone.

He also suffered bleeding to both eyes, indicating "significant" head trauma, consistent with "vigorous shaking".

A V-shaped bruise on Leyton's forehead was consistent with an impact from a shod foot, and a pathologist said the extensive bruising and fractures to his groin and genitals was "most likely explained by Leyton having received a heavy blow or blows, such as a stamp or kick to his genital region".

Experts concluded severe internal injuries to his body were caused by punches.

Scans also showed multiple and unsurvivable injuries to the boy's brain and medical life support was withdrawn two days later in hospital.

Dawick claimed Leyton was lying on a duvet on a sofa when he accidentally pulled the cover from underneath the child "with a lot of force" sending the child "flying" and crashing to the hard floor, causing the injuries.

The jury came back with guilty verdicts in under three hours.

Stephen Meadowcroft QC, defending Dawick, said Leyton's death was not a result of prolonged "torture" but a "complete loss of control" by Dawick, with the fatal attack "very, very quick".

Flynn, who suffered battered women's syndrome, was bailed to be sentenced at a later date following the preparation of pre-sentence reports.

The judge told the jury of seven women and five men that due to the "harrowing" nature of the evidence they were excused from jury service for the next 10 years.